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PHILADELPHIA (AP) | The term “flash mob” used to mean online-organized groups having street-corner pillow fights or sidewalk dance-offs, but in Philadelphia they’ve turned violent, and the reasons are as tough to pin down as where the next one will be.

At least five such mobs have gathered in the City of Brotherly Love in the past year, all leaving property damage or injuries in their wake, after hundreds of teenagers communicating by text message, Twitter, Facebook and other social media spread the word for everyone to show up at a designated spot.

Police Commissioner Charles H. Ramsey said Wednesday that he has been talking to law enforcement officials in other cities to see whether any have encountered similar problems. He doesn’t think Philadelphia is unique, but was not aware of another municipality that has seen teen mobs of the same size and frequency.

The local mobs involve mainly middle-schoolers and high-schoolers - not the college-and-older crowd that typically take part in nonviolent flash mobs. And the younger kids don’t gather with the intention of performing something silly en masse; generally, the only goal is to gather at a specific location, though one was sparked as rumors spread of a brawl at a downtown mall.

“Social phenomena have to start somewhere,” said Frank Farley, a psychologist and professor at Temple University. “It could be some kind of coincidence of fate that will get picked up later on elsewhere.”

In the most recent mob, on Saturday night, witnesses estimated as many as 2,000 teenagers thronged the narrow sidewalks, blocked traffic, jumped on cars and roughed up bystanders around South Street, a 10-block strip of bars, clothing stores, pizzerias and cheesesteak joints that has long been a hangout for teens and 20-somethings.

There were three arrests and multiple assaults, and many stores and restaurants closed early amid fears of trouble brewing.

Mr. Farley, an expert on risk-taking and thrill-seeking personalities, said the flash mobs have attributes that many teens would find attractive.

“This kind of thing I could see catching on across the country the more it’s publicized,” he said. “It’s easy to do, it’s thrilling, it’s fun, and they can turn on the TV the next day and say, ‘I was there.’ ”

Philadelphia is stepping up enforcement and ratcheting up penalties against juveniles in an effort to put an end to the roving groups wreaking havoc in downtown business and tourist districts.

“The lion’s share of these kids don’t have any nefarious intent,” said Deputy Police Commissioner Richard Ross, “but if you have a group of 1,000, 2,000 kids and only 25 are disruptive, that’s still unacceptable.”

Mayor Michael Nutter said Wednesday that if the “stupidity” continues, he will permanently move up the city’s curfew for minors, currently 10:30 p.m. on weekdays and midnight on weekends, as police and businesses have suggested. The mayor didn’t say what those earlier curfews might be.

Police are also monitoring social-networking sites and message boards for early warning of potential disturbances, and the city transit authority is monitoring subways for any unusual spikes in riders all headed to one area.

Those arrested in the three mobs that gathered since February face felony charges - a departure from previous clashes in May 2009 and December 2009, when misdemeanor counts were filed.

Family Court Judge Kevin Dougherty this week found 28 teenagers guilty of felony rioting for incidents on Feb. 16, when 150 teens stormed through a downtown department store, fighting and breaking items, and on March 3.